1. Field
One embodiment of the present invention relates to a perpendicular magnetic recording medium and magnetic recording/reproduction apparatus for use in, e.g., a hard disk drive using the magnetic recording technique.
2. Description of the Related Art
Recently, a demand has arisen for increasing the capacity of a hard disk drive, and the recording bit size is more and more decreasing as the recording density increases. To form a large-capacity hard disk medium, it is necessary not only to decrease the recording bit size but also to improve the recording/reproduction characteristics, i.e., reduce noise generated from the medium. The main cause of the medium noise is presumably a zigzagged domain wall in the bit boundary portion. One method of reducing the noise generated from the bit boundary portion is to form a clearer recording bit boundary. This makes it possible to reduce the magnetic interaction between recording bits, and accurately perform recording/reproduction in each individual bit.
It is disclosed by, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-77122, an example of the means for improving the recording/reproduction characteristics is a technique in which in a perpendicular magnetic recording medium formed by sequentially stacking at least a nonmagnetic underlayer, magnetic layer, and protective layer on a nonmagnetic substrate, the magnetic layer is made of ferromagnetic crystal grains and a nonmagnetic grain boundary mainly containing an oxide, the nonmagnetic underlayer is made of a metal or alloy having the hexagonal closest packed crystal structure, and a seed layer made of a metal or alloy having the face-centered cubic crystal structure is formed between the nonmagnetic underlayer and nonmagnetic substrate. This technique is particularly characterized in that the seed layer is made of a metal selected from Cu, Au, Pd, Pt, and Ir, an alloy containing at least one of Cu, Au, Pd, Pt, and Ir, or an alloy containing Ni and Fe. This technique can orient the (111) plane as the closest packed face of the face-centered cubic structure as the seed layer, and can orient the nonmagnetic underlayer formed on the seed layer and having the hexagonal closest packed structure along the (002) plane. This makes it possible to improve the crystal orientation of the recording layer having the same hexagonal closest packed structure as the nonmagnetic underlayer, and obtain a perpendicular magnetic recording medium having good magnetic characteristics.
When the crystalline seed layer having the face-centered cubic structure is used, however, the crystal orientation improves, but the crystal grains become difficult to downsize because the grain size of the seed layer is reflected on the nonmagnetic underlayer.
As disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-327006, for example, there is a technique having tried to improve the recording/reproduction characteristics and increase the thermal decay resistance by using a perpendicular magnetic recording medium in which at least a soft magnetic underlayer, first nonmagnetic underlayer, second nonmagnetic underlayer, perpendicular magnetic recording film, and protective film are formed on a nonmagnetic substrate, the first underlayer is made of Pt, Pd, or an alloy of at least one of Pt and Pd, and the second nonmagnetic underlayer is made of Ru or an Ru alloy. In particular, a Pt alloy or Pd alloy obtained by adding another element to Pt or Pd can be used in the first underlayer in order to downsize the crystal grains. Favorable examples of the additive element are B, C, P, Si, Al, Cr, Co, Ta, W, Pr, Nd, and Sm. This technique has tried to improve the crystallinity of the second nonmagnetic underlayer and magnetic recording layer by particularly adding C.
Unfortunately, although the crystal orientation and recording/reproduction characteristics improve by the addition of the additive to Pt or Pd, grains are observed in the first nonmagnetic underlayer, i.e., the layer maintains the shape of a crystal grain as described in the embodiment. In this case, the grain size in the first nonmagnetic underlayer imposes limitation and makes it difficult to further decrease the grain size in the magnetic recording layer.